What Does Oil Do?
Your engine is made up from hundreds of moving metal parts that constantly rub up against each other when the ignition is on. Oil helps lubricate these parts,allowing them to move without causing unwanted friction.
Oil also helps disperse the heat, allowing your engine components to stay cool and work properly.When oil is running low,various signs will appear to let you know,so you can take action before it's too late. 

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Symptoms Of Low Oil:
1)Low oil level – dipstick: This is one of the simplest ways to determine whether you’re low on engine oil. Just wait until the engine cools down, pop the hood,locate the dipstick, remove it, and check the reading. If it turns out that the oil level is low, you should seek professional service immediately.

2)Flickering Check Oil light: Lots of drivers stay on the lookout for an illuminated Check Oil icon. However, a flickering Check Oil icon can be just as significant! This could indicate an issue with the oil or an issue with the sensor.Either way, it’s a problem, and somebody should check it out right away.

3)Pungent burning smell: The odor of burnt oil is unmistakable, and so is its cause. When there isn’t enough oil, the engine’s parts start to heat up due to friction, which can burn the oil. This is an extremely pressing sign, and you should act with urgency. Our team of expert automotive technicians can help.

4)Loud knocking sound: Bang! Bang! Bang! Bang! Bang! This is the final, and most serious, symptom, just short of a dangerous total engine failure. Without oil, your engine rods will wear down, and they’ll eventually break. The noise comes from a broken rod that’s knocking around inside there.

Oil consumption has many reasons.Let’s go through the most common ones quickly:

Reasons for Engine Oil Consumption
1)Engine design: Some engines consume oil by design. From day 1. The consumption can be as high as 1 liter per 1000 kms (cca. 1.7 quarts per mile). This depends on the make and model of the engine.

2)Driving style: RPM affects oil consumption.The higher your usual RPM is the more oil will be consumed. That’s because you put extra pressure on the seals and gaskets and some of the oil finds its way around and gets burned away in the combustion chamber.

3)Seal and gasket condition:
The more worn they are the more oil will be consumed. What happens is that the oil finds its way around the seals and it gets burned away in the combustion chamber or it simply leaks out.

4)Higher-than-normal engine temperature: The hotter your engine runs the thinner your oil will be. Once again it will be simpler for a thin oil to reach the combustion chamber and get mixed and burned with the fuel.

5)Too-low viscosity motor oil: If you use an oil with a viscosity below the recommended by the engine manufacturer than you will experience oil consumption.By viscosity I do not mean SAE viscosity only. The HTHS(High Temperature High Shear)viscosity also counts.

6)Low quality motor oil with high volatility:
I mention this as the last one because this is the least common reason.If the oil’s volatility is very high than it could be noticeable in the oil consumption rate.

There are also numerous other reasons including but not limited to high oil level,the damage or clogging of different engine parts, Worn or broken piston rings,worn or damaged camshaft bearings,worn or damaged connecting rod bearings,clogged "PCV" valve and the fuel contamination of the oil.

http://www.oilspecifications.org/articles/what-are-the-reasons-of-oil-consumption.php

https://www.normreevesvw.com/symptoms-of-low-engine-oil-irvine-ca

https://autojosh.com/symptoms-low-engine-oil-reasons-engine-oil-consumption/